The overall objective of the work proposed is to determine how IgA production is regulated in mice. We plan to determine whether the differentiation of bone marrow derived (B) cells into IgA producing plasma cells is regulated by special subsets of thymus-derived T cells, uniquely adapted for interaction with IgA precursor B cells, either by recognition of such cells or by preference for the same lymphoid organs, i.e., the gut-associated lymphoid tissue (GALT), or whether any T helper (Th) and T suppressor (Ts) cells of the appropriate carrier specificity will do. We shall examine the possibility that special T cells or non-lymphoid elements of GALT, specifically Peyer's patches, provide a differentiation signal, necessary for the differentiation of B cells, bearing mu and/or delta chains, to IgA production. We shall probe the question of whether there can be a secondary IgA response (i.e., whether there are IgA memory B cells) and whether there are T dependent and T independent antigens capable of stimulating IgA production. We shall attempt to determine how the selective migration of different populations of B and T lymphocytes into different lymphoid organs is controlled.